1. Field of the Invention
The invention is in the field of micromachined z-axis rate gyroscopes and in particular to gyroscopes with a multidirectional drive-modes.
2. Description of the Prior Art
With their dramatically reduced cost, size, and weight, MEMS gyroscopes potentially have a wide application spectrum in the aerospace industry, military, automotive industry and consumer electronics market. The applications are diverse, including high performance navigation and guidance systems, automotive safety systems like yaw and tilt control, roll-over protection and next generation airbag and antilock brake systems. A wide range of consumer electronics applications include image stabilization in video cameras, virtual reality products, pointing devices, and game industry. Miniaturization of gyroscopes also enable higher-end applications like micro-satellites, microrobotics, and even implantable devices to cure vestibular disorders.
Almost all existing micromachined rate gyroscopes operate on the vibratory principle of a single proof mass suspended above the substrate. The proof mass is supported by anchored flexures, which-serve as the flexible suspension between the proof mass and the substrate, making the mass free to oscillate in two orthogonal directions. FIG. 2a is a diagram of a prior art gyroscope showing the drive direction (x-Axis) and the sense direction (y-Axis). The overall dynamical system is simply a two degrees-of-freedom (2-DOF) mass-spring-damper system, where the drive direction is excited by the electrostatic drive forces, and the sense direction is excited by the rotation-induced Coriolis force.
Decomposing the motion into the two principle oscillation directions, the drive direction x and the sense direction y, the simplified equations of motion becomem{umlaut over (x)}+cx{dot over (x)}+kxx=Fd mÿ+cy{dot over (y)}+kyy=−2mΩz{dot over (x)}.  (1)where Fd is the drive-mode control force that provides constant-amplitude drive mode oscillations. The final term, 2mΩz dx/dt, in the equation is the rotation-induced Coriolis force, which causes dynamic coupling between the oscillation axes, and is used for angular rate measurement.
In most of the reported micromachined vibratory rate gyroscopes, the proof mass is driven into resonance in the drive direction by an external sinusoidal force, which are generally the electrostatic forces applied by comb-drive structures. When the gyroscope is subjected to an angular rotation, the Coriolis force is induced in the y-direction. If the drive and sense resonant frequencies are matched, the Coriolis force excites the system into resonance in the sense direction, as well. The resulting oscillation amplitude in the sense direction is proportional to the Coriolis force and, thus, to the angular velocity to be measured.
To achieve the maximum possible gain, the conventional gyroscopes are generally designed to operate at or near the peak of their resonance curve. This is typically achieved by designing and electrostatically tuning the drive and sense resonant frequencies to match. Alternatively, the sense-mode is designed to be slightly shifted from the drive-mode to improve robustness and thermal stability, while sacrificing gain. The drive and sense mode matching (or near-matching) requirement in vibratory gyroscopes renders the system response very sensitive to variations in system parameters, e.g. due to fabrication imperfections and fluctuations in operating conditions, which shift the drive or sense resonant frequencies. For the devices packaged in vacuum to enhance the sensitivity, the bandwidth of the resonance peaks is extremely narrow; leading to much tighter mode matching requirements. Extensive research has focused on design of symmetric drive and sense-mode suspensions for mode-matching and minimizing temperature dependence.
However, especially for lightly-damped devices, the mode-matching requirement is well beyond fabrication tolerances; and none of the symmetric designs can provide the required degree of mode-matching without feedback control. Furthermore, as the modes are matched more closely, the mechanical interference between the modes becomes more significant, resulting in operation instability and drift.
In order to suppress coupled oscillation and drift and to minimize the resulting zero-rate drift, various devices have been reported employing independent suspension systems for the drive and sense modes. The approach of decoupling drive and sense modes led to the first integrated commercial MEMS gyroscopes produced by Analog Devices. We have previously reported gyroscope systems that offer improved robustness by expanding the degree-of-freedom of the dynamical system with the expense of sacrificing response gain. We also reported increased-DOF gyroscope systems with decoupled modes to minimize quadrature error.
However, the scarce capabilities of photolithography and micro-fabrication processes, and the resulting inherent imperfections in the mechanical structure significantly limits the performance, stability, and robustness of MEMS gyroscopes. Thus, fabrication and commercialization of high-performance and reliable MEMS gyroscopes that require picometer-scale displacement measurements of a vibratory mass have proven to be extremely challenging.
The limitations of the photolithography-based micromachining technologies defines the upper-bound on the performance and robustness of micromachined gyroscopes. The mode-matching problem and the quadrature error due to the resulting fabrication imperfections are the two major challenges in MEMS gyroscope design.